Does the PVC injection molding machine need to be cleaned?

1. Why is it necessary to clean a PVC injection molding machine?

PVC Thermal Degradation
PVC is very sensitive to temperature. If it remains at processing temperatures (typically 170-210°C) for extended periods, it decomposes and releases hydrogen chloride (HCl) gas. This gas:

It is highly corrosive: It can severely corrode metal components of the injection molding machine, such as the screw, barrel, and mold, shortening the equipment's lifespan.

It is harmful to the human body: It is a strong irritant and can damage the respiratory tract and eyes.

It can contaminate subsequent products.
If you switch to other materials (especially transparent or light-colored products) after producing PVC without thoroughly cleaning it, residual, degraded PVC black specks and yellow streaks can seriously contaminate the new product, resulting in large quantities of scrap.

Preventing Material Incompatibility
PVC is incompatible with many other plastics (such as PC, PET, and PMMA). Mixing these can lead to serious quality issues such as embrittlement, delamination, and surface defects.

2. How should I properly clean a PVC injection molding machine?

There is a set of standard operating procedures for cleaning PVC injection molding machines. The core principle is "cleaning when stopping the machine and cleaning when changing materials".

(1). Stop and clean at the end of production (short-term or long-term shutdown)

Standard operating procedures:
Close the material gate: Stop adding new PVC raw materials to the barrel.
Eject the barrel: Continue to inject until the screw can no longer move forward, and eject as much PVC material in the barrel as possible (can be used as a purging injection station).

Switch to purge compound:
Use a special PVC purge compound. This purge compound usually has good thermal stability and cleaning ability, and can effectively wrap and remove residual PVC.
If there is no special purge compound, HDPE (high-density polyethylene) or ABS can be used as a transitional purge material. Because they are incompatible with PVC, they can effectively push it out.

Repeated injection until clean: Add purge compound, repeatedly inject and extract glue until there is no black or yellow impurities of PVC in the ejected material strip.

Cooling down and shutting down:
Short-term shutdown: After cleaning, the temperature can be set to around 150°C for insulation. Long-term shutdown: After cleaning, the barrel must be filled with general-purpose plastics such as PS or PP as "sealing material" before turning off the heating and power. This can isolate the air and prevent oxidation and rust on the metal surface.

(2). Cleaning when switching materials

When switching from PVC to other materials, or from other materials to PVC, thorough cleaning is required.

Switching from PVC to other materials (such as PC, PMMA):
Transition materials must be used: First, use HDPE or ABS to thoroughly clean the PVC.
Then use the target material: After confirming that it is clean, add the target material such as PC or PMMA for production. This can avoid contamination and product defects caused by incompatibility.

Switching from other materials to PVC:
Usually, PP or HDPE is used to clean the previous material first.
Then add PVC material. Since PVC is corrosive, it will clean up some residues by itself, but the initial injection part should be treated as waste.

3. Cleaning Precautions

Temperature Control: During cleaning, first lower the barrel temperature to the lower limit of PVC processing temperature (e.g., 160-180°C). Once the material is clean, adjust the temperature to the temperature of the new material.

Personal Protection: During the cleaning and blasting process, HCl gas may be released. Operators should wear masks and goggles, and ensure good ventilation in the workshop.

Specialized Cleaning Compound: For manufacturers who frequently process PVC, investing in a specialized PVC cleaning compound is the most cost-effective option. It provides the most effective and safe cleaning performance, protecting equipment.